This invention relates to CMP (xe2x80x9cchemical mechanical planarizationxe2x80x9d) materials and specifically to CMP materials comprising alumina powders as the abrasive.
CMP is a process that is used to prepare semiconductor products of great importance in a wide range of electronic applications. Semiconductor devices are typically made by depositing a metal such as copper in spaces between non-conductive structures and then removing the metal layer until the non-conductive structure is exposed and the spaces between remain occupied by the metal. The demands placed on the abrasive are in many ways in conflict. It must remove the metal but preferably not the non-conductive material. It must remove efficiently but not so quickly that the process cannot be easily terminated when the desired level of removal has been reached.
The CMP process can be carried out using a slurry of the abrasive in a liquid medium and it is typical to include in the slurry, in addition to the abrasive, other additives having a xe2x80x9cchemicalxe2x80x9d effect, including complexing agents; oxidizing agents, (such as hydrogen peroxide, ferric nitrate, potassium iodate and the like); corrosion inhibitors such as benzotriazole; cleaning agents and surface active agents.
The formulation is usually intended to operate within a narrow range of pH values since the pH significantly affects the material removal rate and therefore the predictability of the process. There is however a problem in that the pH of a formulation tends to drift with time such that a formulation has a definite xe2x80x9cshelf lifexe2x80x9d. In practice many formulations need to be made up just before use, which is inefficient.
The present invention provides a method of formulating a CMP formulation that gives a slurry with a remarkably stable pH for a very protracted period allowing consistent and predictable performance during that period.
The present invention provides a process for the production of a CMP slurry comprising alumina and an iodate oxidizing agent which comprises the steps of:
a. acidifying an alumina slurry and an iodate solution to pH levels between 2 and 5 that differ by less than one pH point; and
b. mixing the slurry and the solution in the desired proportions to produce a CMP formulation.
It is surprisingly found that this process causes the drift of pH over time to be very significantly reduced. The acid selected can be an inorganic acid such as the commonly used nitric acid, or more preferably it can be an organic acid such as formic acid, acetic acid, glycine, maleic acid, citric acid, proprionic acid or succinic acid and the like. The use of acetic acid is found to be a very effective and economic alternative with little in the way of handling problems attendant on its use.
It is further found that the stabilization of pH is enhanced further if the mixture, whether mixed before pH adjustment or pH adjusted prior to mixing as taught above, is treated to an elevated temperatures, that is from about 70xc2x0 C. to about 250xc2x0 C. (if conducted under hydrothermal conditions, for example in an autoclave), for a period from about 1 and often as long as 8 hours. The time of treatment varies inversely as the temperature of the treatment such that higher temperatures require shorter treatment times. Boiling at atmospheric pressure is effective if carried out for a period of from 5 minutes to eight hours or more preferably from 15 to 45 minutes is generally sufficient. To avoid loss of the dispersant medium and acid during the boiling it is preferred to boil under reflux. Enhancement of the pH stability can often be achieved in shorter times if carried out under hydrothermal conditions such as one minute at 180xc2x0 C. It is believed that such heat treatments are also effective to minimize pH drift in polishing slurries not dedicated to CMP applications and which therefore do not comprise an oxidizer component.
The alumina slurry preferably comprises alpha alumina particles with an average primary particle size of from 0.02 to 0.5 micrometer though slurries with primary particle sizes from 0.01 to 1 micrometer are believed to benefit also from the process outlined herein.
The iodate oxidizing agent can be any one of those typically used in CMP applications such as alkali metal iodates and particularly potassium iodate.